Enlarge the size of your cell.
You data may be to large to display in the current size
This may also happen in a cell type mismatch. Right-click on the affected cell(s) and select Format cell. Select the appropriate data type (number or currency) plus the number of decimal points you want to see.
To shortcut the resize process, select the column by clicking the column letter. Now, double-click the right-hand divider between cells. This will auto-fit the cell size to the cell contents.
The above resize trick works with a range of columns. Select all the columns you want resized and double-click any of the column dividers.
Another problem you may face is exactly as you describe. The solution, however, is to click on a little triangle on a top corner of the cell. Look out for it; I don’t recall what causes it but floating your pointer over the triangle and/or the cell should clue you in.